There are known strain gauges of the type that comprises a sensitive element, such as a tensoresistor element, which interacts with a movable support through which a signal, indicative of the value of strain, is applied to the strain gauge. The sensitive element converts the signal to an electric signal to be subsequently processed by an automatic system for processing and recording strain values (cf., e.g., FRG Application No. 2,647,780 cl. G. 01 B 7/16, published Apr. 27, 1978).
One of the crucial problems in connection with strain gauges of this type is the temperature compensation due to changes of the values of deformation depending on the ambient temperature. There are different solutions to the problem (cf., e.g., USSR Inventor's Certificate No. 301,514, cl. G 01B5/30, published in "Discoveries, Inventions, Industrial Designs and Trade Marks" Gazette No. 14, 1971).
In some cases use is made of thermoresistor compensation elements placed in series with the sensitive elements.
It is also common to use mechanical thermocompensation elements of a material possessing a linear thermal expansion coefficient roughly equal to that of the article subjected to strain measurements. There are cases when the mechanical thermocompensation elements and the article subjected to strain measurements are of the same material.
In such strain gauges the mechanical termocompensation element is rigidly attached to a second support and coupled through a tensoresistor sensitive element to a first support which is movable (cf., e.g., British Patents No. 1,469,644, cl. G 01B 5/30 and No. 1,443,771, cl. G 01 B5/30).
All the above gauges are checked and graduated prior to installation on an article to be tested. After a gauge is installed in place, its graduation and checking are either totally impossible or require much time. For example, a strain gauge may be checked by connecting it into one of the arms of a bridge measuring circuit and by comparing the signal appearing across the diagonally opposite pair of junctions of the bridge to the original and supplemented signal of the same gauge.
In fact, the method boils down to checking the electric circuit alone; no provision is made for checking the entire signal circuit, including the mechanical parts of the strain gauge.